An inverter-driven rotating electric machine system that uses an inverter to drive a rotating electric machine has been developed and widely used. In the inverter-driven rotating electric machine system, the inverter performs a switching operation to convert DC voltage to pulse voltage, and supplies the pulse voltage to the rotating electric machine via a cable. The rotating electric machine is driven by the pulse voltage.
In a conventional high-voltage rotating electric machine, particularly in order to prevent partial discharge that could occur near an end of a core of a stator coil and generation of heat, an electric-field-reduction system is provided in many cases on a surface of the coil near the end of the core of the stator. The electric-field-reduction system is a combination of a low resistance layer, which is led out of a stator core slot, and an electric field reduction layer, which is formed in such a way as to partially overlap with the low resistance layer.
Meanwhile, in the inverter-driven rotating electric machine system, reflected waves are generated due to an impedance mismatch between the inverter, cable, and rotating electric machine. The reflected waves are superimposed on the pulse voltage. As a result, in a unit between the cable and the rotating electric machine, or particularly in a unit where the cable and the rotating electric machine are connected, high voltage noise, or so-called inverter surge, could occur.
If the pulse voltage including the inverter surge (which will be referred to as inverter pulse voltage) occurs repeatedly, the partial discharge that occurs during operation at a commercial frequency and the generation of heat become even larger at the above-described core-end stator coils (which will be referred to as stator coil ends). Even on the electric-field-reduction system, partial discharge and heat are generated to such a degree that the reliability is hampered. Finally, the reliability of the stator coils could be significantly reduced.
The partial discharge and the generation of heat are dependent on the surface potential of the electric-field-reduction system (Refer to Non-Patent Document 1). Accordingly, there is an increasing need for a technique for accurately measuring the surface potential of the electric-field-reduction system in the case that the inverter pulse voltage would occur.